1. Field of the Invention
The present invention relates to a method of producing an aluminum thin-film useful for constructing a variety of nanodevices, including a magnetic recording media built into various types of magnetic recording devices, such as external memory devices for computers. The invention additionally relates to a method of fabricating alumina nanohole arrays, and a method of manufacturing magnetic recording media, using the aluminum thin-film and the alumina nanohole array thus produced.
2. Description of the Related Art
The recent trend toward high-density memory in magnetic disks has been accompanied by a shift in the magnetic recording method from conventional in-plane recording (longitudinal recording) to perpendicular recording. The development of perpendicular recording methods has dramatically improved the recording density. With in-plane recording, the upper limit in the recording density was about 100 Gbits/in2, whereas today the recording density has surpassed 400 Gbits/in2.
However, the limit in the recording density attainable in the first-generation of simple perpendicular recording methods is 400 Gbits/in2. The reason is as follows. Increasing the recording density requires that the bit size be made smaller. Yet, when the bit size is decreased, bit deterioration due to thermal fluctuations, i.e., random inversions of magnetization, has a tendency to arise. Satisfying formula (1) below is a necessary condition for avoiding bit deterioration due to such thermal fluctuations.
                              [                      E            ⁢                                                  ⁢            1                    ]                ⁢                                                                                                                          K              u                        ⁢            V                    kT                >        60                            (        1        )            
In formula (1), Ku is the uniaxial magnetic anisotropy constant, V is the volume per bit of the magnetic recording layer, k is the Boltzmann constant, and T is the absolute temperature. The right side of formula (1) is called the thermal stability index.
That is, to overcome the instability of thermal fluctuations arising from the inevitable decrease in volume V as the bit size is made smaller, it is necessary to increase the thermal stability index. If the operating temperature is fixed, the uniaxial magnetic anisotropy constant Ku value must be increased.
Ku is a constant that depends on the magnetic material and for which a relationship like that in formula (2) holds.
                              [                      E            ⁢                                                  ⁢            2                    ]                ⁢                                                                                                H          c                =                                            2              ⁢                              K                u                                                    M              s                                -                                    M              s                        ⁡                          (                                                N                  z                                -                                  N                  y                                            )                                                          (        2        )            
Here, Hc is the magnetic coercivity, Ms is the saturation magnetization, and Nz and Ny are demagnetization coefficients in, respectively, the z direction and the y direction.
It is apparent from above formula (2) that the magnetic coercivity Hc has a positive correlation with Ku. That is, if, as mentioned above, a magnetic material having a large Ku is selected in order to overcome thermal fluctuations, the magnetic coercivity Hc representing the magnetic field strength which reverses magnetization also increases, making it more difficult for the magnetic head to reverse the magnetization, and in turn making information more difficult to write to the magnetic recording medium. Such challenges as (1) the “decrease in volume associated with higher density”, (2) the “long-term stability of recordings to thermal fluctuations”, and (3) the “difficulty of recording due to a higher magnetic coercivity He” are all complexly intertwined, presenting a “trilemma” that makes it impossible, by the mere extension of earlier approaches, to arrive at a solution for achieving higher density recording.
A number of methods have been proposed recently for resolving such a trilemma. Of these, one promising method is thermally assisted magnetic recording.
Thermally assisted magnetic recording resolves the above trilemma by overcoming (3) the “difficulty of recording due to a higher magnetic coercivity Hc” without addressing the other two challenges. Specifically, at the very moment that information is written with a magnetic head to a magnetic recording medium which uses a high Hc material, the magnetic recording medium is heated by a short period of irradiation with light so as to lower, for a short time, the Hc of the recording medium, thereby making it possible to carry out writing even with a low magnetic field. The long-term stability to thermal fluctuations can be ensured by cooling once more to the read temperature within a short enough time that bit deterioration due to thermal fluctuations does not occur.
In this way, research and development has begun on a thermally assisted magnetic recording system prototype as a next-generation perpendicular recording system, and there are indications that it may be theoretically possible to achieve a recording density in excess of 1 Tbits/in2. Thermally assisted magnetic recording theoretically has a large potential, and is thus a promising candidate for next-generation perpendicular recording systems (see, for example, Japanese Patent Application Laid-open No. 2006-12249; Japanese Patent Application Laid-open No. 2003-45004; and Fujitsu, 58, No. 1, pp. 85-89 (2007)).
However, careful investigations targeted at practical development have revealed a number of difficulties.
For example, if the magnetic recording medium is a continuous medium like the magnetic films that have hitherto been used, difficulties that may occur during such heating include (1) a rise, as well, in the temperature of neighboring bits and (2) the failure to achieve sufficient magnetic separation.
Solving the above difficulties is possible if the bits can be thermally isolated at the same time that they are magnetically isolated. Attempts are thus being made to employ the concept known as “bit patterned media” (BPM) targeted at the magnetic isolation of all of the bits.
Several approaches have been proposed for bit patterned media, one of them being bit patterned media which utilize alumina nanoholes (alumina=aluminum oxide). Although patent applications relating to alumina nanoholes have been filed for many years (see, for example, Japanese Patent Application Laid-open No. S61-261816; Fujitsu, 58, No. 1, pp. 90-98 (2007); Heisei 15 (2003) Kanagawa Academy of Science and Technology, The Masuda “Nanohole Array” Project Research Report; and Abstracts of the Spring 2004 Meeting of the Electrochemical Society of Japan, item 1 A28), a technical reassessment of BPM which utilize alumina nanoholes is taking place today. This is due, in particular, to the shift from conventional in-plane magnetic recording to perpendicular magnetic recording and to the intense desire for magnetic recording media having recording densities in excess of 1 Tbits/in2.
Alumina nanoholes differ from other BPM-forming methods in that they have the following characteristics (a) to (c).
(a) A mold of metal or the like in which a regulated array of innumerable projections have been created by microfabrication is pressed against an aluminum thin-film to form micropits which then serve as the starting points for anodization wherein alumina nanoholes are formed in a self-organizing manner.
(b) The alumina nanoholes are a group of nanoholes that exist in a controlled array and are separated by alumina. By filling these nanoholes with a magnetic material using an electroplating process or the like, a magnetic recording medium in which one nanohole corresponds to one bit can be formed.
(c) Because the alumina in which alumina nanoholes have been formed is a material that is nonmagnetic and has a very low thermal conductivity, the individual bodies of magnetic material filled into the respective nanoholes can be both magnetically and thermally isolated, resulting in an ideal structure for thermally assisted BPM processes.
Yet, even in alumina nanoholes having such outstanding characteristics, a number of difficulties have emerged from further investigations.
One such difficulty has become apparent in the course of efforts to increase the purity of the aluminum starting material for aluminum film formation and reduce impurities with the aim of forming nanoholes having a diameter of about 20 nm with minimal defects. That is, as indicated also in Abstracts of the Spring 2004 Meeting of the Electrochemical Society of Japan, item 1 A28, the higher the purity of the aluminum starting material, the greater the tendency for crystal grain growth to proceed in the aluminum film thus formed, resulting in a marked loss in the surface smoothness of the film and thus making it difficult to uniformly form pits of several nanometers in depth as starting points for alumina nanoholes.
To cite one example, when an aluminum thin-film having a thickness of 100 nm is formed at room temperature on a silicon substrate using an aluminum target of 99.99% purity, the arithmetic average roughness (Ra) attains a value of 2 nm. At an Ra value of 2 nm, the surface has a maximum roughness (Rmax) of generally 15 nm. Even if an attempt were made to form pits having a depth of several nanometers at intervals of 20 nm on an aluminum thin-film with such an irregular surface, the excessively large surface irregularities would make it impossible to obtain uniform pits.
A way has thus been sought to suppress crystal grain growth during film formation so as to improve surface smoothness and reduce the surface irregularity on an aluminum thin-film so that, for example, Rmax is 3 nm or less. For example, Abstracts of the Spring 2004 Meeting of the Electrochemical Society of Japan, item 1 A28, describes an attempt to suppress crystal grain growth by an alloy process involving the addition of impurities to aluminum. This reference describes the addition of from 2 to 10 wt % of magnesium to form Al—Mg alloys. Moreover, at the moment, when nanohole formation efforts by most research laboratories are being targeted (1) at the formation of pits using a mold or die on a small-diameter substrate having a diameter of about one inch, or (2) at the formation of pits in local regions on a 2.5-inch substrate, harmful effects due to the use of Al—Mg alloys have not been noted in particular.
However, as is explained in greater detail later in this specification, the inventor has discovered that, in a crystal grain growth suppressing method involving an alloy process in which impurities are added to aluminum, the Vickers hardness of the aluminum thin-film increases, as a result of which the pressure needed to form the pits that serve as the starting points for the alumina nanoholes is three to four times that needed for pure aluminum. There is thus a tendency for wear and damage of the metal projections to occur, predictably shortening the life of the mold or die. An additional concern is the larger molding equipment required for the pit-forming operation.